1. Field of the Invention
The invention relates generally to tape drive for tape transducer assemblies and, more particularly, but not by way of limitation, it relates to improvements in speed control and regulation of capstan drive motors.
2. Description of the Prior Art
The prior art includes numerous types of drive circuit and assembly which have been used for tape movement in tape transducer assemblies. Such prior approaches to tape drive have utilized both AC and DC types of motor, and each has its inherent advantages and disadvantages. Synchronous AC motor drive systems can provide the required constancy of speed, but these are subject to the limitations of fixed mechanical ratios that usually determine an exact speed. While such AC systems are relatively immune to drift caused by voltage, temperature and load parameters, slippage due to belts or friction components often attribute variables. Utilization of an AC motor drive system also severely limits the ability to select different speeds, and the inherent nature of the synchronous motor makes continuously variable systems impractical.
Direct current motors require a regulatory system employing some form of feedback, and either voltage or current feedback systems may adequately control tape speed if the load is constant. However, variable loading will cause fluctuations, a period of which is complicated by the time constant of an attendant regulator circuit. Phase locked loop systems for DC motor speed control help to overcome many of the problems of DC regulation and provide more precise and convenient selection of speed. Some such systems achieve the effect of an AC synchronous motor, but do not drive the motor directly with the reference frequency, relying instead upon indirect means or synchronization. The loop configuration generally does not include more than the motor itself, and thereby neglects other aspects of the drive system such as belts, pulleys and other mechanical parameters.